This invention relates to a method and an apparatus for removing the heat of polymerization evolved during a polymerization carried out while the liquid polymerization mixture is stirred.
By the term "heat of polymerization" used herein we intend to refer to both the heat of a polymerization reaction, i.e. the heat evolved due to the polymerization reaction of monomers, and the heat of agitation, i.e. the heat evolved by stirring the liquid polymerization mixture. Removal of the heat of polymerization with a high efficiency using the known methods is difficult, due to problems accompanying these methods. Methods for the removal of the heat of polymerization hitherto known in the art are broadly divided into the following two types. The first type involves the utilization of a heat transfer medium to which heat is transferred from the polymerization mixture due to thermal conduction. The second type involves evaporation of the volatile liquid by boiling the polymerization mixture whereby latent heat of vaporization is removed.
The first type method is performed, for example, by providing a jacket on the outer periphery of a polymerization reactor, or a coil or draft tube submerged in the polymerization mixture inside a polymerization reactor, and passing a cold heat transfer medium through the jacket, coil or draft tube. This method is not advantageous for the following reasons. When the polymerization mixture increases in viscosity, the polymerization mixture is liable to form a stagnant layer in close proximity to the wall, through which heat is transferred and which is maintained at a reduced temperature by the heat transfer medium, and the polymer is liable to be deposited on such a wall. The stagnant layer of the polymerization mixture and the deposited polymer reduce the efficiency of heat removal from the polymerization mixture. Therefore, it has been hitherto proposed to scrape off the deposited polymer or remove the stagnant layer from the wall. However, this requires an intricate and expensive scraping apparatus, and minute materials produced by the wearing out of the apparatus are incorporated into the polymer product, which leads to a reduction of quality of the product.
The second type method is performed, for example, by boiling the polymerization mixture thereby evaporating a volatile liquid, e.g. the monomer in the case of bulk polymerization, or a mixture of the monomer, a solvent and optionally a boiling accelerator in the case of solution polymerization, and condensing the vapor in a reflux condenser provided outside the reactor, or by flash evaporating the volatile liquid outside the reactor. These two methods are advantageous in that the efficiency of heat removal is high.
However, the former method, i.e. that involving the step of reflux condensation, possesses the following defects. The polymer is liable to be deposited on the inner walls of the reactor and the condenser due to a large amount of bubbles and entrainments in the vapor. Thus, in order to prevent the bubbles and entrainments from entering into the condenser, it becomes necessary to employ a large volume polymerization reactor so as to ensure an increased vacant space above the liquid level inside the reactor. This leads to reduction in the efficiency of polymer production per unit volume of the apparatus. Furthermore, in the case of a continuous polymerization, since the inner pressure of the reactor should be maintained at a low level, so as to ensure boiling of the polymerization mixture, firstly, it is difficult to maintain the head from the reactor to the discharge pump at a level sufficient for smoothly discharging the polymerization product from the reactor and, secondly, undesirable cavitation occurs in the discharge pump due to the boiling of liquid and this makes it difficult to smoothly discharge the polymerization product.
The latter method, i.e. that involving the step of flash evaporation by using an evaporating means placed outside the reactor, can be employed for a continuous polymerization because the inner pressure of the reactor can be maintained at a high level. However, this method also involves some problems. That is, the polymer is liable to be deposited on the inner walls of the reactor and the flash evaporator, and it becomes difficult to deal with the polymerization mixture because of increase in viscosity due to the increase in the polymer content and due to the reduction in the temperature of the polymerization mixture caused by flash evaporation. Furthermore, the flash evaporator is expensive and intricate.
Therefore, it is a primary object of the present invention to provide a method wherein heat of polymerization is removed with improved efficiency and without the above-mentioned defects of the known methods.
Another object is to provide an apparatus for removing the heat of polymerization which is neither intricate nor expensive and possesses none of defects of the known apparatuses.
Other objects and advantages of the present invention will be apparent from the following description.